This invention relates to transistor amplifiers and, in particular, to an improved transistor amplifier which efficiently utilizes an operating voltage, amplifies a large input signal with satisfactory fidelity and has minimal output impedance so as to reduce operating losses.
Conventional field effect transistors of the type used in the prior art are characterized by their pentode-type dynamic characteristics. These prior art field effect transistors (FET's) are not well-suited for amplifying circuits wherein minimal cross-over distortion is required. Accordingly, an improved FET has been proposed which is formed of vertical-type junctions so that it exhibits triode-type dynamic characteristics. This improved FET has a relatively low output impedance, on the order of about 10 ohms, as well as low signal distortion so that it finds ready application in the output stages of audio power amplifiers. Furthermore, this improved FET having triode-type dynamic characteristics exhibits superior switching qualities over the prior art FET so that it can be advantageously used in a single ended push-pull (SEPP) amplifier without accompanying undesired cross-over distortion.
Unfortunately, when the gate-source voltage of the improved FET is reduced to zero, the resultant saturation condition prevents proper amplification of a large input signal. Consequently, because of this saturation characteristic of the improved FET, its response to a large input signal is even less desirable than that of a conventional bipolar transistor.
Now, if bipolar transistors are used to amplify large input signals, for example, when used in a push-pull amplifier, cross-over distortion normally is present. This distortion is most undesirable. Also, when a bipolar transistor is conducting, its resistance, for example, the collector-emitter resistance (also known as the OH resistance) is relatively high. This high resistance of the conducting bipolar transistor results in an inefficient use of the operating voltage which is applied thereto. Stated otherwise, the bipolar transistor exhibits a low source voltage utilization rate. Therefore, when a large output is required, the attendant cross-over distortion and inefficient use of the operating voltage often determines that a plurality of FET's are to be used in an attempt to avoid the aforenoted poor saturation characteristics of an individual FET. Unfortunately, the use of plural ones of these devices to avoid such poor saturation characteristics is extremely expensive.